Recently, development is remarkable on what is called digital audio technology in that an analog signal of voice or the like is recorded and reproduced by converting into a digital signal. For example, a PCM processor using a mechanism of a VTR and a tape, a compact disc, more recently a digital audio tape recorder of a rotary head method (hereinafter is referred to as R-DAT), and moreover a digital VTR (hereinafter is referred to as D-VTR) which records not only a voice signal but also a video signal by converting into digital, are put to practical use. Furthermore, there is a product which is made to record only the audio signal by digitalizing in an exclusive area contiguous to an analog video signal track in the conventional VTR. All the VTR, R-DAT or the like are of what is called helical scan type which generates a PCM signal by sampling an audio or video signal, and records and reproduces by forming oblique tracks on a tape by a rotary head. Consequently, a relative speed by which the rotary head traces a track on the tape is always substantially constant in variable speed reproduction which is slower than the normal reproducing speed such as what is called JOG-MODE and VARI-MODE (variable speed playback mode). Therefore, a pitch (frequency) of reproduced sound does not vary as by the conventional method which records and reproduces in analog way by forming a track in the length direction of a tape and by using a stationary head even if a reproducing speed is varied.
Hereafter, the VTR is elucidated as an example.
In a VTR or the like for broadcasting use, an image signal of one frame is divided and is recorded in plural tracks in many cases. Therefore, in the case of the variable speed reproduction, a perfect image can not be reproduced, unless all the tracks of a set in which the image signal for the one frame is recorded are reproduced. For this reason, auto-tracking control (hereinafter is referred to as AT control) which controls the mounted height of a reproducing head by using a piezo-electric device is performed generally in order to correctly trace a desired track in variable speed reproduction. In many cases of the AT control, for example, the position of a target track is presumed on the basis of a control signal recorded in a control track for capstan servo which is formed along the length of the tape and a FG signal from the capstan, and moreover a tracking control is performed on the basis of the output level of a reproduced signal from the rotary head.
On the other hand, when slow speed reproduction is performed in an apparatus having no AT control such as the R-DAT, the rotary head sometimes traces on a track which is different in azimuth, and a reproduced output is severely reduced, and data error occurs during the period.
FIG. 16 is a figure showing a PCM signal output which is reproduced in the case that both the VTR and R-DAT perform reproduction of 1/4 times speed. Since the AT control is performed in the case of the VTR as mentioned above, the rotary head tracks four times on the same track. Consequently, the PCM signal in the same frame is repeatedly reproduced four times. There is, as what has been put into practical use, a D-VTR for broadcast application standardized by SMPTE which is called D1 and D2 formats, as an example which is repeatedly reproduced in variable speed reproduction as mentioned above.
In the case of the R-DAT, since the gap width of a reproducing head is wider than the track on the tape, and data which is reproduced by a head of inverse azimuth is made to a signal format which can be interpolated with each other even if one side is of error, the reproduced sound of the same frame is continuously reproduced by several times.
However, in the digital audio apparatus using the rotary head as mentioned above, reproduced sound of a normal pitch which is punctuated with a periodic time is repeatedly reproduced in variable speed, particularly in low speed reproducing operation. Compared with what the frequency of the reproduced sound varies responding to tape speed like the conventional analog type, there is a problem that search of the head position of a desired program is difficult in editing operation in particularly business use, broadcast use, and operability is very poor. For this reason, in a similar manner in the apparatus mounted with the digital audio, an apparatus which can reproduce a voice signal having high quality at a frequency corresponding to the reproducing speed in the variable speed reproduction is required with emphasis.
Accordingly, a method which introduces the signal of the same frame which is repeatedly reproduced in the low speed reproducing operation into a memory, and read out at a clock frequency corresponding to the reproduction speed is elucidated in the Japanese laid open patent gazette 61-3368, as a prior art. The reference is designated as Reference 1. In the Reference 1, a basic concept as to the variable speed reproducing method using a memory is shown. However, there was not made clear and concrete presentation as to a method which is applicable to not only the D-VTR but also the R-DAT, and detects renewal of the same frame signal which is repeatedly reproduced, and means which writes surely in the above-mentioned memory only the signal of renewed one frame. Namely, in an apparatus in which the AT control is not performed, and in the event that the number of repetition of the same frame which is reproduced is gradually varied by arbitrary variation of the reproducing speed, it is uncertain whether renewal detection of the frame and writing in the memory are securely performed. Moreover, since the PCM signal read out from the memory is reduced in sampling rate in accordance with the reproducing speed, in the obtained state, quantization noise is generated in an audible frequency band. Therefore, filtering process is applied to an intermittent PCM signal read from the memory, and an interpolating filter is required to return to the original sampling rate. However, there is a problem that the interpolating filter is not referred in the Reference 1, and a high quality reproduced sound can not be obtained in the given condition.
On the other hand, a prior art in which it is intended that a reproduced digital signal is once stored in the memory apparatus of a large capacity, and the stored data Is intermittently read by operating a jog dial, and an effect which is identical with the variable speed reproduction is obtained by interpolating between the read samples is elucidated in the National Technical Report: Vol. 26, No. 6, December 1980, p. 932 -p. 934. The document is designated as a reference 2.
The reference 2 shows an interpolating method of a variable speed reproducing apparatus of a digital audio tape recorder using a stationary head for business use. The prior art as to interpolation in the second reference is elucidated hereafter. First, a reproducing speed of PCM voice is varied in accordance with the revolution speed of the dial. Thereby, reproduced interval of the PCM sample varies. If the reproduced interval of the PCM sample is interpolated by "LEVEL HOLDING", waveform distortion significantly increases. Therefore, linear interpolation is performed. Namely, a linear increase is assumed between two samples A, B, and a sample which is newly generated by using a multiplier and an adder interpolates. However, although the above-mentioned method has an advantage that it is realizable by a simple circuit, in case of a low speed reproduction there is a problem that the interpolated waveform is different from the input PCM sample and includes a waveform distortion, and the sound quality is severely damaged, since the input PCM sample is linearly interpolated as described above.